Ground Effect Flying Boats (GEFB) or Wing In Ground-effect (WIG) craft operate very efficiently when flying close to the water surface on a dynamic air cushion. Lift is increased due to higher, more uniform pressure created in the cushion under the wing, while parasite drag is reduced due to low velocity in the air cushion. Induced drag is greatly reduced by near elimination of wing tip vortices due to close proximity of the wing tips with the ground. However, these craft have historically been overpowered. One of the reasons for the excessive power is the high drag encountered in the water-borne mode. Numerous tests have shown that the peak "hump" drag in the water-borne mode is approximately 2.7 times the drag present during low speed flight in ground effect. A second reason for high power requirement is the inherently low propulsive efficiency of air propellers and jets at low vehicle speed. For example, a standard fixed pitch propeller designed for maximum efficiency at cruise speed will achieve only about 20% propulsive efficiency at the speed corresponding to the hump drag. A variety of take-off aids designed to reduce hydrodynamic drag are seen in the prior art.
One popular method is the use of Power Augmented Ram Wmg In Ground-effect (PARWIG). The well known aerodynamicist Alexander Lippisch utilized this technique in his U.S. Pat. No. 3,190,582 (1965). By mounting the air propeller ahead of the wing leading edge a sufficient distance, he was able to achieve a ram effect under the wing to produce a slight static lift at low speed, thus reducing wave making drag somewhat. The reported power required to achieve takeoff of the 550 lb. craft was 15 hp, while the minimum cruise power in ground effect was measured at 4 to 5 hp. For free flight, the power requirement rose to 18 to 20 hp. Thus, even with this take-off aid, the power required to take off was three times the power required to sustain flight in ground effect.
Another PAR WIG craft is the subject of U.S. Pat. No. 4,712,630 by Blum (1987). This craft utilizes a diffuser duct to smoothly spread the airflow from a forward mounted propulsor to the wide space under the lifting body. The object is to uniformly "fill" the space in an attempt to control backflow. Lateral diffusion of the airflow as proposed will reduce the velocity in approximate proportion to the increased lifting area exposed. Unfortunately, since dynamic pressure is proportional to the square of the velocity, the pressure is reduced substantially for a given area increase. As a result, the diffuser duct actually reduces net lift. In addition, the very small diameter propulsor that is required to fit in the neck of the diffuser duct must operate at very high velocity in order to develop adequate thrust, and therefore has relatively low propulsive efficiency. A further disadvantage of the design is the use of a pilot controlled damper to block flow exiting under the body in order to create static lift pressure. The damper can create an unstable flight condition at the moment of take-off when airflow escapes under the body trailing edge. Sudden downward deflection of the airflow at the trailing edge causes the lift force to suddenly shift to the rear, placing demands on the pilot during the critical take-off transition when balance is essential.
Another, more effective method of reducing hydrodynamic drag is the use of an air pressurized cavity under the hull. The cavity is a void space interposed between the hull and the water typically bounded by the hull sides, the water surface, and moveable gates at the fore and aft ends of the cavity. Air is pressurized by a fan and fed to the cavity. The air pressure imposed between the hull and the water lifts the hull, reducing wave making drag. Additionally, leakage of pressurized air between the water and hull planing surfaces greatly reduces frictional drag. An application of this "hovercraft" technique is presented in U.S. Pat. No. 3,903,832 granted to Ishida et al (1975). The rear half of the craft utilizes a pressurized cavity, powered by a fan separate from the main propulsion fan. The forward half of the craft is designed as a ram wing to take advantage of dynamic lift at high forward speed. Unfortunately, the lift fan is required at all speeds, does not contribute to propulsion, and consumes large amounts of power. Furthermore, location of the static lift cavity behind the craft CG makes it impossible to lift the forward hull portion which results in high wave making drag.
Another method for achieving a pressurized static lift device is depicted in U.S. Pat. No. 4,151,893 by Peter Mantle (1977). Several pressurized cavity designs are proposed, most of which use the same engine for both propulsion and lift. However, all methods shown depend on the added complexity of flow diverting ducts, dampers, and gates. These devices reduce flow efficiency and add cost. Additionally, dependence on these devices compromises safe operation during the critical transition from water-borne to air-borne mode.
Yet another pressurized cavity lift system is presented in U.S. Pat. No. 5,636,702 by Gordon Kolacny (1997). A common compressor means is used for both propulsion and pressurization of a cavity to generate lift at low speed. The cavity configuration requires the use of movable confinement flaps in order to transition from the low speed hover mode to a streamlined flight mode. Dependence on the flaps for control during the critical transition period places extra demands on the pilot, compromising safety. Additionally, the compressor means utilizes high pressure fans or turbines which develop higher pressures and velocities than are desired for optimum propulsive efficiency.
In summary, the shortcomings of existing air cushion take-off aids for ground effect craft are the following:
a) Power augmented ram wing lift systems are inefficient, have inherent backflow, and develop low pressure.
b) Pressurized cavity lift systems most often are designed with auxiliary high pressure lift fans and motors that consume extra power and add weight.
c) Methods used to effect transition from static to dynamic lift modes include movable jets, diffusers, and/or control dampers. These devices add complexity and cost, and often compromise safety.
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